Plastic materials are light in weight and excellent in toughness, and are easy to dye. Due to these properties, plastic materials have been frequently used for various types of optical materials, particularly spectacle lenses, in recent years. Performance required for optical materials, particularly optical materials for spectacle lenses, includes physical properties such as low specific gravity, chemical and thermal properties such as low yellowness index and high heat resistance, as well as mechanical properties such as high strength and so on, while the required optical performance includes high transparency, high refractive index and high Abbe's number. High refractive index allows a reduction in lens thickness, while high Abbe's number reduces chromatic aberration in lenses. However, higher refractive index gives lower Abbe's number, so that attempts have been made to improve both refractive index and Abbe's number. Among these attempts, typically listed are processes using an episulfide compound as shown in Patent Document 1. Also, there has been proposed an optical material having a refractive index of 1.73 or higher, which is composed of a polyepisulfide compound and an inorganic compound, as shown in Patent Documents 2 and 3.
On the other hand, lenses which have become thinner due to high refractive index are required to have high strength more than ever before in terms of safety, etc. For example, when applied to a frame which is called a rimless frame, the lenses should be drilled to make holes and hence are required to have enough strength not to generate cracks and/or breaks during drilling. Moreover, to withstand use in a rimless frame, the lenses are required to have enough strength not to be broken even when the frame is bent. At the same time, the lenses are also required to have heat resistance, because it is routine to provide plastic lenses with hard coating for surface protection and heat should be applied during such hard coating formation.
Against this backdrop, attempts have been made to satisfy three requirements, i.e., high refractive index, high strength and heat resistance at the same time. Among these attempts, typically listed are processes using the following four compounds: a compound having an episulfide group, an inorganic compound having a sulfur atom and/or a selenium atom, a thiol compound forming a thiourethane bond, and an isocyanate compound, as shown in Patent Documents 4 and 5. In addition, to obtain a homogeneous and transparent optical material having a refractive index of 1.73 or higher, it is known that an inorganic compound having a sulfur atom and/or a selenium atom and a compound having an episulfide group are subjected to pre-polymerization reaction. When a thiol compound and an isocyanate compound are each added to this pre-polymerization reaction product, rapid polymerization will occur, so that any homogeneous and transparent optical material cannot be obtained or only an optical material of very thin thickness can be obtained. In Patent Document 5, a product of pre-polymerization reaction between a thiol compound and an isocyanate compound is added to the above pre-polymerization reaction product to thereby avoid rapid polymerization, although the resulting casting composition is highly viscous and therefore difficult to cast.
Thus, in the preparation processes shown in the above documents which cause rapid polymerization or generate a highly viscous product, it has been difficult to practically prepare an optical material having a refractive index of 1.73 or higher, which is composed of the following four components: an inorganic compound having a sulfur atom and/or a selenium atom, an episulfide compound, a thiol compound and an isocyanate compound. For this reason, there has been a demand for the development of a process for producing a lens which has a refractive index of 1.73 or higher and is provided with strength and heat resistance.